metamorphic
Gneiss (Granitic Gneiss)
Gneiss (from German Gneis, related to 'spark')
Hardness: 6-7 on Mohs scale; Color: Banded pattern of pink (feldspar), white/gray (quartz), and black (biotite/amphibole); Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Banded pattern of pink (feldspar), white/gray (quartz), and black (biotite/amphibole); Luster: Dull to vitreous; Structure: Foliated with distinct bands (gneissic banding); Specific Gravity: 2.6-2.9.
Formation & geological history
Formed through high-grade regional metamorphism of pre-existing igneous rocks (like granite) or sedimentary rocks. It is subjected to intense heat and pressure deep within the Earth's crust, causing minerals to recrystallize and segregate into bands.
Uses & applications
Used widely as a crushed stone in construction (road base, concrete aggregate), dimensional stone for flooring and facades, and as decorative landscaping or gravestones.
Geological facts
Gneiss contains some of the oldest rock formations on Earth, such as the Acasta Gneiss in Canada which is roughly 4 billion years old. The term 'gneiss' has been used in English since at least 1757.
Field identification & locations
Identify in the field by looking for 'gneissic banding'—alternating layers of light and dark minerals that do not easily split (unlike schist). Commonly found in the cores of mountain ranges and ancient continental shields.
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